speed is constant, so frequency varies as 1/wavelength. If wavelength increases frequency decreases and vice versa
energy varies with frequency –
the higher the frequency, the shorter the wavelength and the higher the energy the lower the frequency, the longer the wavelength and the lower the energy
electromagnetic spectrum –
in order of increasing wavelength, (decreasing energy):
Xrays, gamma rays ( < 0.2 m)
ultraviolet radiation (UV) (m)
visible light (0.4 < < 0.7 m)
infrared radiation (0.7 < < 100 m)
microwaves (mm, cm)
radio waves (m)
radiation is emitted by everything
the wavelengths of radiation emitted by an object depend on the object’s temperature
the hotter the object, the shorter the wavelength of radiation emitted
hotter objects radiate more total energy per unit area than cold objects
Stefan-Boltzman law: E ~ T4 , E = energy per sec per unit area. (T = temperature in K)
Wien’s law: max= 2900/T. max = wavelength where most radiation is given off.
A “Black Body” is a perfect absorber and emitter of radiation (It absorbs and re-emits all the energy it receives). The earth and sun behave as black bodies (and thus obey the Stefan-Boltzman and Wien’s laws)
solar spectrum – peaks in visible (sun’s surface temperature ~ 6000K – use Wien’s Law)
earth’s radiation – peaks in far infrared (earth’s surface temperature ~ 288K – use Wien’s Law)
The earth emits the energy at infrared wavelengths (because the temperature of the earth is ~288K, the wavelength where the maximum radiation occurs is ~ 10 m – from Wien’s Law)
(Can use infrared detectors to look for warm bodies.)
Gases that absorb in the solar spectrum block incoming radiation from the sun and heat up the atmosphere – these are primarily oxygen (O2) and ozone (O3). They are important because they prevent the highest energy ultraviolet radiation from reaching the earth. Absorption by ozone heats up the stratosphere, absorption by oxygen heats up the thermosphere.
The gases that absorb in the infrared spectrum block outgoing radiation from the earth and re-radiate it back to the earth’s surface, warming the surface. These are greenhouse gases.
albedo of earth-atmosphere ~ 30% (20% reflected by clouds, 10% by earth’s surface and scattered back by gases)
(albedo = % of radiation striking a surface which returns back)
(scattering = deflection of light by gases and particles – gives atmosphere its blue color)
~20% of incoming solar radiation is absorbed by gases, dust and clouds in the atmosphere
~50% absorbed at the earth’s surface (and heats earth)
Earth absorbs ~ 50% of solar energy which reaches the top of the atmosphere. It then radiates this energy back into the atmosphere at infrared (IR) wavelengths. (The solar radiation is at short wavelengths because the sun is hot. The earth’s radiation is at long wavelengths because the earth is much cooler.)
Some gases in the atmosphere absorb the outgoing IR radiation at certain wavelengths ) – nitrous oxide, methane, water vapor, carbon dioxide. (These are greenhouse gases.)
There is an ‘atmospheric window’ between 8 and 13 m where most radiation emitted from the earth reaches space without being absorbed.
Earth’s energy budget –
Solar energy heats the surface of the earth. The earth is in equilibrium (it is not continuously heating up or cooling down) so it gets rid of the same amount of energy as it takes in. (There is a balance of incoming and outgoing energy over the whole earth’s surface over the year, but at any particular location at any particular time there will be a net intake or outflow of energy – eg. during the day the incoming energy exceeds the outgoing energy and vice versa at night.)
The earth loses energy by:
conduction (contact between surface air and surface of earth) and convection (resulting upward motion of warm air)
evaporation (provides latent heat of evaporation)
Most of the radiation emitted by the earth is absorbed in the atmosphere by greenhouse gases and clouds. Some of this energy will be re-radiated back to the surface of the earth. This raises the temperature of the earth to an average of ~ 288K (15 ºC).
The greenhouse effect is a natural phenomenon which is necessary to sustain life on earth (otherwise it would be too cold). The reason that we are concerned with rising CO2 levels is that we are enhancing the natural greenhouse effect and thus warming up the earth. If the earth warms up by even 1 or 2 degrees our climate will change dramatically and ice caps could melt, sea levels rise, and coastal cities flood.